Well the problem is if you all taken the battery out more than 10-20seconds without replacement within that 10seconds then the electrical charge built up inside the conductor in the N900 will be depleted and which lead to storage setting lost during that time.

Due to N900 designed flaws, there is no emergency battery for the bios to keep the date and time setting intact like other devices. eg: symbian.

The N900 cannot keep the date and time setting for more than 10seconds (sometimes can be 20seconds) after the battery is taken out or depleted completely. That is why when the battery is low to 4% threshold, the device will shut it self down and going in a coma to save it's setting from being lost.

If you changing battery, you will have to do it very quickly within the 10seconds time frame, otherwise N900 will result in amnesia symtom.

I hope my explaination is clear and helpful. I hope the software is smart enough to preserve the setting and syncronise it with the provider in the future.